Plural of Fish

Please could someone settle an arguement between me and my housemate.
I am currently sitting on the lawn outside the house, using the
wireless network, appealing for help. If this can be resolved, perhaps
he will let me in so i can go to bed. :-) (Here in Oxford, UK it is
bedtime)

What is the plural of Fish, is it Fish or Fishes? What do you think

Many thanks

(the real) Jim Grady